1. Field of the Invention
The present invention relates to the field of semiconductor optoelectronic devices comprising an active region in strip form.
The present invention applies in particular, but not exclusively, to semiconductor lasers.
2. Background Information
The most popular laser structure in use today is formed from a double heterojunction.
As shown in attached FIGS. 1 and 2, semiconductor lasers formed from a double heterojunction generally comprise, on a semiconductor substrate 10, an active layer 14 produced from a low energy gap semiconductor material which amplifies reflected light by stimulated emission (recombination of electron/hole pairs resulting in an emission of light at an energy close to that of the material's energy gap), surrounded respectively on each side of its main faces 13, 15, by layers 12, 16 produced from higher energy gap semiconductor materials.
Semiconductor lasers are generally fabricated on GaAs or InP substrates 10. For lasers employed in telecommunications operating at wavelengths of 1300 nm and 1500 nm, substrate 10 is produced in InP. For lasers operating at wavelengths of 850 nm and above, up to 630 nm (for compact disk lasers for example), substrate 10 is produced in GaAs.
The interface between active layer 14 and subjacent layer 12 constitutes the p-n junction of the laser.
The two layers 12, 16 of higher energy gaps ensure the transverse confinement of the structure, i.e. perpendicularly to the junction formed between active layer 14 and the higher energy gap adjacent layer 12.
Active layer 14 is less than 1000 nm thick (typically 200 nm). Layers 12 and 16 which surround it are thicker, and are generally over 1000 nm thick (typically 1500 and 2000 nm).
Active zone 14 also has a higher refractive index than surrounding zones 12 and 16 For example, for a double heterostructure laser produced on an InP substrate 10 emitting at 1550 nm, the GainAsP active zone 14 is 200 nm thick and has an index of 3.5, and the optical and electrical confinement layers 12 and 16, which are in InP, are approximately 1500 nm thick and have an index of 3.2.
The light emitted in active region 14 by the recombining of carriers, holes and electrons is guided by the low indexes of confinement layers 12 and 16. Preferably, a large overlap is sought between active gain zone 14 where the photons are emitted and guide layers 12 and 16.
Active region 14 is also confined both longitudinally and laterally.
Longitudinal confinement is obtained by cleaving the crystallographic structure making up active region 14 along faces 110 or 1-10 so as to form two reflectors.
The beam therefore generally propagates in direction 100 which corresponds to the largest dimension of active layer 14. This dimension ranges from 0.05 mm for short lasers to 1 mm for long lasers.
The width of active layer 14, defined by lateral confinement, is generally in the range 1000 to 2000 nm.
Different techniques has been proposed to ensure the lateral confinement of active layer 14, i.e. to define an active zone in strip form that is narrower than substrate 10.
Five known lateral confinement techniques are shown schematically in FIGS. 3 to 7.
According to the first known technique shown in FIG. 3, a rectangular strip is etched in the base substrate, the active zone then being epitaxed in this strip. This technique makes it possible to obtain a channeled substrate planar laser.
According to the second known technique shown in FIG. 4, an entire laser structure is produced in a single growth. Next, optical lateral confinement is provided by etching the entire structure to approximately 200 nm above the active zone around a strip. The strip is approximately 1000 nm to 3000 nm wide so as to provide a stable mode. An oxide layer is then deposited to electrically insulate everywhere except on the strip. The contact above the strip is used to supply current. The low index of the insulation close to the active zone (at 200 or 300 nm) above this zone prevents the optical mode from spreading laterally outside the strip. The advantage of this structure is that it can be produced without restarting epitaxy, and is known as the strip structure (ridge waveguide laser).
According to a third known technique shown in FIG. 5, a V-shape is etched into an epitaxed layer. A double heterostructure is then fabricated by restarting epitaxy using LPE (liquid phase epitaxy) above this V-shaped etch, respecting the rule that n zones are placed inside the V in front of the p zones located outside the V and vice versa in order to obtain good lateral electrical confinement. This is facilitated by the fact that when epitaxy is restatted, the layers are thicker at the center of the V than at the extremities. This gives these layers a crescent shape (hence the name buried crescent laser).
According to a fourth know technique shown in FIG. 6, the structure known as the DCPBH structure (Double Channel Planar Buried Heterostructure Laser) is obtained by etching two channels, one on each side of a strip which serves as an active zone. N and P zone epitaxy is restarted around the strip on the etched channels to ensure the laser's optical and electrical confinement. Epitaxy is restarted by LPE.
According to a fifth known technique shown in FIG. 7, double epitaxy is used to produce a structure known as a BRS structure (Buried Ridge Structure). The first epitaxy is used to grow a buffer layer and then the optical and electrical confinement zones surrounding the active zone. Next, a 1000 to 2000 nm strip is defined by etching the material around this strip and restarting epitaxy with a material of low index (the same as the confinement zones above and below the active zone).
Although the different known techniques described above have rendered great service, they are not, however, totally satisfactory.
Note, first, that the different index guided structures shown in FIGS. 5, 6 and 7 require epitaxy to be restarted which weighs down the technology and increases fabrication time.
Furthermore, the structures shown in FIGS. 3 and 4, which are made without restarting epitaxy, and whose optical guiding is achieved by gain, and therefore low, are not very stable in operation.